Today, many forms of information are sent from information sources, such as television content providers, to receivers, such as televisions in people's homes. Thus, an example of such information is digital television (DTV) information. Transmitting digital information typically involves converting the digital information to an analog signal and modulating an RF (radio frequency) carrier frequency's amplitude and/or phase using the analog signal, and sending the modulated signal over a propagation medium.
Referring to FIG. 1, a communications system 200 includes a transmitter 202 and a receiver 204. The transmitter 202 and the receiver 204 have respective antennas 206, 208, here shown externally to the transmitter 202 and the receiver 204, although the antennas 206, 208 may be considered to be parts of the transmitter 202 and the receiver 204. The transmitter 202 is configured to send information over a propagation medium, here a terrestrial broadcast system, to the receiver 204. Transmitting information over the propagation medium introduces signal distortion caused by noise w(t) (e.g., static), strength variations (fading), phase shift variations, multiple path delays, etc. Multiple path delays result from the transmitted signals taking different paths between the transmitter and receiver through the propagation medium, e.g., due to reflections off buildings 210 and/or relay through repeater stations 212. Different paths of a transmitted signal p(t) result in different gains g0, g1, g2, and different delay times d0, d1, d2 that cause replicas g1p(t−d1), g2p(t−d2) of the signal p(t) to arrive at different times at the receiver 204 (like an echo) compared to the directly-transmitted signal g0p(t−d0). The received signal r(t) is a combination of the directly-transmitted signal and/or the replicas, if any. Multi-path distortion results in inter-symbol interference (ISI) in which weighted contributions of other symbols are added to the current symbol.
In addition to distortion and noise from the propagation medium, front-end portions of the receiver and transmitter may also introduce distortion and noise. The presence of distortion, noise, fading and multi-path delays introduced by the overall communication channel (transmitter, receiver and propagation medium), can cause digital systems to degrade or fail when the bit error rate exceeds a threshold and overcomes the error tolerance of the system.
Equalization
Equalization is employed at the receiver to help compensate for distortion of the transmitted information. The transmitted information in a digital system is pulse shape filtered symbols having discrete levels of amplitude and/or phase. The digital receiver uses a slicer to make hard decisions as to the value of the received symbol. A slicer is a decision device responsive to the received signals at its input, which outputs the nearest symbol value from the constellation of allowed discrete levels. A slicer is also known as a nearest element decision device. To the extent that a symbol is received at a level that differs from one of the allowed discrete levels, a measure of communication channel error can be detected.
The receiver uses an equalizer that is responsive to the detected error to mitigate the signal corruption introduced by the communications channel. It is not uncommon for the equalizer portion of a receiver integrated circuit to consume half of the integrated circuit area.
An equalizer is a filter that attempts to match the inverse characteristics of the communication channel. If the transmission characteristics of the communication channel are known or measured, then the equalization filter parameters can be determined. After adjustment of the equalization filter parameters, the received signal is passed through the equalizer, which compensates for the non-ideal communication channel by introducing compensating “distortions” into the received signal which tend to cancel the distortions introduced by the communication channel. In most situations such as in HDTV broadcasting, however, each receiver is in a unique location with respect to the transmitter. Accordingly, the characteristics of the communication channel between the transmitter and any given receiver are not known in advance, and may even change with time. In those situations where the communication channel is not characterized in advance, or changes with time, an adaptive equalizer is used. An adaptive equalizer has variable parameters that are calculated at the receiver. The adaptive equalizer attempts to adjust the equalizer filter parameters in order to restore received signal quality to a performance level that is acceptable.